By inkjet technology is meant a printing method with which a printed image is generated by the targeted launch or the deflection of ink droplets. These methods have been used in the graphic market for several years. Water and solvent-based inks as well as UV-hardening inks are primarily used. Water- or solvent-based inks usually dry by evaporation of the solvent. UV-hardening inks dry via a hardening process which is initiated through UV-radiation. The UV-hardening inks are thereby irradiated shortly after application on a substrate with a UV-lamp. Since the UV-hardening inks harden independently of the evaporation of their solvents, the printed substrates can be further processed substantially earlier and therefore enable considerably shorter process times or significantly rapid processes.
Another advantage of UV-hardening inks is an earlier manipulation. Contrary to water or solvent based inks which dry by evaporation, UV-hardening inks do not dry up in the print head and thereby save extensive cleaning of the head. UV-hardening inks also stick well to very different materials, among other things, on synthetic materials. The use of UV-hardening inks has therefore obtained a proper place in inkjet technology.
Usual UV-hardening ink formulations are based on acrylates. They can be hardened with UV light only to become a color coat when photoinitiators are contained in the formulations. The photoinitiators are excited by UV light and break down in radicals which release the polymerisation reactions of the acrylate oligomers. Mainly radicals with a defined structure are thereby built up which also react further to the polymers. It must be provided, however, that the photoinitiators do not lie in shadow zones since no excitation will there occur due to an absence of UV radiation with the result that the formation of chain starting radicals does not occur.
A further shortcoming of the UV technology is that residual portions of the reaction products always remain undefined since side reactions producing a large number of various products take place in addition to the main reactions. Said by-products are quite multifaceted and are only partially available in minimal quantities in the ppm range so that they are analytically very difficult to identify.
Fission or by-products can also appear directly in case of decomposition of the photoinitiator, which migrate from inks or color coats. A migration of undefined by-products is, however, not acceptable for a some applications, for example, in the food packages area. This is aggravated by the fact that the portions of the by-products mentioned can vary according to process parameters such as light intensity, speed and layer thickness.
Another shortcoming is that the UV-hardening ink can penetrate on porous substrates, i.e., it can be sucked up by the substrate. It thereby evades irradiation with UV-light, which in turn leads to an incomplete reaction of the acrylate oligomers. The incomplete reaction generates other by-products than in the case of a complete hardening. The by-products generally generate undesirable odours. The odor is generally more intensive the less the coat has hardened.
The undesirable by-products are usually not identifiable. They are therefore not accessible to classification according to the specifications of the use of print colors in the food sector. Since the evaluation of risks is not possible, UV-hardening inks have only very rarely been used for printing food packages and in other health-critical sectors such as, for example, for printing toys.
In order to use the advantages of the UV-hardening inks, photoinitiators are known which, due to their structure, have a reduced migration potential. This is achieved by producing larger molecules with a smaller migration tendency out of the original smaller photoinitiator molecules by derivatisation with bulky substituents. While this may avoid rapid migration, it does not avoid the tendency of undesirable by-products. Its reactivity is also reduced by the higher molar mass of the photoinitiator. The result in many sectors is an unacceptable reduction in processing speed. The derivatised photoinitiators are therefore only classified as suitable for indirect food contact.